Much of the remaining crude oil resources are heavy oil or tar, both of which can also contain increased amounts of deleterious components such as sulfur. The low mobility of this oil makes its production difficult without some external stimulation such as heat. The most widely used method of thermal oil recovery is steam injection. A well-designed steam injection project is very efficient in recovering oil, however, its applicability is limited in many situations. Field performance and simulation studies have shown that very deep reservoirs, small thickness of the oil-hearing zone, higher reservoir pressures and reservoir heterogeneity detrimentally impact the performance of steam injection to a significant extent.
There are various technologies available to extract heavy oil. These technologies differ in several important ways: cold (ambient temperature) vs. thermal processes. Examples of cold production processes for viscous heavy oil and oil sands include: conventional production, water flooding, cold heavy oil production with sand (CHOPS), solvent injection, water injection alternating with gas injection (WAG), inert gas injection, and pressure pulsing. Examples of thermal production processes for viscous heavy oil include: steam flooding, cyclic steam stimulation (CSS), steam assisted gravity drainage (SAGD), and underground combustion.
As highlighted above, a variety of enhanced oil recovery methods have been developed and applied to mature and depleted reservoirs in order to improve the efficiency of recovery methods. The processes involved with heavy oil production often require excessive external water supplies for maintaining water pressure as well as steam generation, washing, and other steps. Management and disposal of the wastewater presents challenges and costs for the operators. In addition, production of heavy oil requires a substantial amount of energy for removing the heavy oil from the ground, processing it, and transporting it off-site.